Even if the butt SN has been removed, it will also be stamped in several other places, most notably on the bottom of the barrel and the rear face of the cylinder. Unfortunately, the only legally recognized SN location is somewhere on the frame and those other locations don't count. There is no simple way to make yours legal even though there are procedures for doing so in special circumstances. The greatest risk is in being arrested for some reason and having that revolver in your possession.

One thing you should do. Remove the grips and see if whoever did the work may have stamped the SN somewhere on the grip frame under the grips. If it is there you are OK.